A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps: melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass; forming an article from tile glass and cooling the glass below its transformation range; crystallizing ("ceramming") the glass article by an appropriate thermal treatment. The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range, followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei.
Use of a glass-ceramic as a biomaterial was initially proposed by W. T. MacCullouch in a publication entitled "Advances in Dental Ceramics" in British Dental Journal, 124:361-365 (1968). The glass-ceramics proposed were from the Li.sub.2 O--ZnO--SiO.sub.2 system for production of dentures.
Subsequently, numerous patents have issued disclosing bio-active glass-ceramic materials. Predominantly, the materials are phosphorus-containing, have an apatite crystal phase, and may contain a second crystal phase and a residual glassy phase. Among materials in clinical use are an apatite-phlogopite material in Europe and apatite-wollastonite materials in Japan. Substantial activity in the United States has been by L. Hench et al. at the University of Florida. This activity has centered on glass, and more particularly on development in vivo of a strong bond between bone and glass. This is said to involve formation of a hydroxycarbonate apatite (HCA) layer.
U.S. Pat. Nos. 4,386,162 and 4,397,670 (Beall) disclose glass-ceramics containing canasite as a predominant crystal phase. Canasite is described as a multiple chain silicate exhibiting an anisotropic, blade-like crystal habit, and having the formula Ca.sub.5 Na.sub.4 K.sub.2 [Si.sub.12 O.sub.30 ]F.sub.4. The canasite-containing glass-ceramics have been found to exhibit exceptional mechanical strength and fracture toughness.
It would, of course, be highly desirable to provide bio-active glass-ceramics that have greater mechanical strength and toughness values than those found in the known bio-active glass-ceramics containing apatite. Therefore, it would be desirable to marry the strength and toughness of the chain silicate glass-ceramics to the bioactivity of the apatite glass-ceramics. It is a basic purpose of this invention to provide this desirable combination.